1. Field of the Invention
This invention relates to the purification of impotable water. More particularly, this invention relates to the purification of impotable water using hypochlorous acid.
2. Description of the Prior Art
For many years chlorine has been recognized as a good disinfectant and in one form or another has been involved in the majority of systems designed to produce potable water. In almost all modern water or sewage treatment plants, for example, chlorine is used to reduce the number of bacteria from the final effluent before it is discharged from the system to a potable water distribution system or into a river or stream. A chlorination plant or system may also provide for removing color, correcting tastes, eliminating odor and suppressing other types of undesirable biological growths. Chlorine is also widely used in the treatment of industrial wastes and waste waters.
Substantial advances in the chemistry of water chlorination have been made in recent decades. This chemistry is known to be relatively complex and various aspects of water chlorination chemistry may be reviewed in "Handbook of Chlorination" by G.C. White (Van Nostrand Reinhold, Second Edition, NYC, 1986).
In this connection, however, it is recognized that chlorine is a good disinfectant when applied to water in the form of a solution of chlorine (Cl.sub.2) gas in water. When the chlorine is added to water it rapidly hydrolyzes to form hydrochloric acid (HCl) and hypochlorous acid (HOCl), the latter partly dissociating to provide hydrogen ions and hypochlorite ions (H.sup.+ OCl.sup.-).
The three forms of available chlorine, namely, molecular chlorine (Cl.sub.2), un-ionized hypochlorous acid (HOCl) and the hypochlorite ion (OCl.sup.-), exist together in equilibrium, the relative proportions of each being determined by the PH value and temperature of the water. These forms of "available" chlorine react relatively rapidly with various types of water components including algae and bacteria as well as certain nitrogen and hydrocarbon compounds.
The "available" forms of chlorine compounds are known to be generally more potent than the "combined" forms, i.e. chloramines, which are produced by reaction with compounds present in the water to be treated. Dissolved molecular Cl.sub.2 is also believed to be a very potent disinfecting agent, but does not exist in any appreciable quantity under equilibrium conditions at the pH of most water bodies. The pH must be reduced to a relatively low value in order to enable molecular Cl.sub.2 to exist in solution in substantial concentrations.
It has been postulated that disinfection can be enhanced by improving sewage-aqueous chlorine mixing and by lowering the chlorine reaction pH. In treating water, the "Operation of chlorination equipment which supplies Cl.sub.2 gas to water operates at partial pressures (vacuum). At the vacuum levels currently being used the maximum solubility is about 5000 mg/1. The upper limit of solubility recommended by all chlorinator manufacturers is 3500 mg/1. This arbitrary figure has been successful in preventing solution discharge systems from being adversely affected by gas pockets in the solution piping and off-gassing at the point of application" (White op cit pp 37-38).
U.S. Pat. No. 4,693,832, issued Sep. 15, 1987 to M.M. Hurst, describes a method of preparing potable water by mixing into semi-finished water an aqueous solution of hypochlorous acid having a pH of between about 3 and about 6 in amounts which provide the water with a free chlorine residual of at least about 0.5 ppm. The aqueous solutions contain between 0.1 and 10 grams of HOCl per liter. The method employs dilute solutions of impure hypochlorous acid having high pH values. These HOCl solutions are stated to provide the water with free available chlorine residuals having improved stability. However, to obtain the required pH range it is necessary to supply a base to the hypochlorous acid solution or employ a method of preparation which will prepare hypochlorous acid solutions having the desired pH.